Check pause method exists for given object

[merkushin]

Changes proposed in this Pull Request

• Check the pause method exists before invoking it

Testing instructions

• Enable the feature flag: add_filter( 'sensei_feature_flag_video_based_course_progression', '__return_true' );
  The preferable place is inside sensei-lms.php, somewhere in the beginning.
• Run npm start
• Enable autopause video setting for a course.
• Add a YouTube/Vimeo/Video block to the course lesson.
• Open the lesson with the video.
• Test auto-pause:
  • Start playing video.
  • Try to change tab/window.
  • Video should be paused.

[m1r0]

Works!